The invention is generally related to optical transmission systems, and especially to the protection used therein, the purpose of which is to make sure that the system can go on working even in failure or error situations.
In optical transmission systems, an optical signal is modulated with an outbound data stream, and the modulated optical signal is applied to optical fiber. In order to increase the capacity of the system, the bandwidth of the data stream can be increased or more wavelengths can be introduced, each of which is modulated with a discrete data stream. The latter method is termed wavelength division multiplexing.
Wavelength division multiplexing (WDM) is an efficient way of multiplying the capacity of optical fiber. In wavelength division multiplexing, several independent transmitter-receiver pairs use the same fiber. FIGS. 1a and 1b illustrate the principle of wavelength division multiplexing, using as an example a system having four parallel transmitter-receiver pairs. Each of the four information sources (not shown in the figure) modulates one of four optical transmitters, each of which generates light at a different wavelength (xcex1 . . . xcex4). As will be seen from FIG. 1a, the modulation bandwidth of each source is smaller than the distance between the wavelengths, and thus the spectra of the modulated signals do not overlap. The signals generated by the transmitters are combined onto the same optical fiber OF in a WDM multiplexer WDM1, which is a fully optical (and often passive) component. At the opposite end of the fiber, a WDM demultiplexer WDM2, which is also a fully optical (and often passive) component, separates the different spectral components of the combined signal from one another. Each of these signals is detected at a discrete receiver. Hence, a narrow wavelength window is assigned for the use of each signal in a given wavelength range. A typical practical example might be a system where the signals are in the 1550 nm wavelength range for example in such a way that the first signal is at wavelength 1544 nm, the second signal at wavelength 1548 nm, the third signal at wavelength 1552 nm and the fourth signal at wavelength 1556 nm. Nowadays a multiple of 100 GHz (approx. 0.8 nm) is becoming the de facto standard for the distance between wavelengths.
Current optical telecommunications systems based on wavelength multiplexing have mainly been point-to-point transmission systems, which are used for high-capacity and long distance links (trunk connections). However, the optical transmission technique is being developed constantly to implement the lower layers of broadband network architectures as entirely optical systems, which can be used for relaying high-capacity information flows entirely optically (that is, with the aid of optical cross-connection and routing).
One of the most important features of the optical network or transmission system is its reliability. In order to improve the reliability, optical telecommunications systems use protection methods, which are used to make standby resources available in failure situations. Optical point-to-point connections use two different basic solutions to implement protection: a so-called 1+1 protection, which is illustrated in FIG. 2a, and a so-called 1:1 protection, which is illustrated in FIG. 2b. In the former method, the traffic is sent onto two separate fibers (cables), both of which lead from a source to a destination (usually along different routes). At the receiving end, one of the fibers is chosen for use. If the fiber in use breaks, the other fiber is simply put into use at the receiving end to replace it. In the latter protection method there are also two fibers (OF1 and OF2) between the source and the destination, but the traffic is sent only onto one fiber at a time. If this fiber in use breaks, the other fiber is put into use both at the transmitting end and at the receiving end. FIGS. 2a and 2b show the connections as unidirectional, but the connection may of course be bi-directional. FIG. 2c illustrates 1+1 protection of a bidirectional connection. The protection methods described above may also be utilized in optical networks.
When using 1:1 protection, the system usually needs communication between the transmission end and the receiving end to co-ordinate the switching at the different ends of the connection. In the case of a unidirectional connection, this need is obvious, since the transmission end must get information about any breaking of the fiber. In the case of a bidirectional connection, this need is due to the fact that the connection may break in one direction only, because signals of opposite transmission directions do not usually use the same components. Thus, e.g. an optical amplifier on the path of one transmission direction can fail. However, the communication required between opposite ends of the connection makes the 1:1 implementation significantly more complex than the 1+1 implementation.
The present invention is in fact concerned with implementation of the described 1+1 protection in an optical transmission system, which may be a point-to-point connection of the kind described above or an optical network, the topology of which may vary in known ways. The signals to be used in the system are typically WDM signals, but systems of one wavelength are also possible, since the solution according to the invention does not depend on what type of system signal is in question, since the protection is nevertheless channel-specific (wavelength-specific) in most cases. Thus, also in the case of a WDM signal the protection is usually implemented in a wavelength-specific manner for one or more wavelength channel signals separated from the WDM signal.
As was described above, two nodes of the optical network are usually connected to one another along two different paths, along a so-called working path WP and along a so-called redundancy path RP (FIGS. 2a . . . 2c). When 1+1 protection is used, the same traffic travels between the nodes along both paths, but at the receiving end the decision is made which path will be listened to (that is, the path which will have its traveled signal connected to the receiver). This decision-making usually takes place so that in a normal situation, when there are no faults in the network, the traffic is received from the fiber chosen to be the working path at each time. When the signal received through the working path breaks due to some trouble occurring in the network or its level falls below the permissible minimum, the redundancy path is selected at least temporarily to be the working path, and the signal received from it is connected to the receiver.
However, such a decision-making-procedure is not always the best possible, when both paths are functioning, but the signal strengths received through them are different. This is due to the fact that if the signal arriving along the redundancy path is sufficiently stronger than the signal arriving along the working path, it may cause significant interference, even if it is not connected to the receiver.
It is the objective of the invention to eliminate the drawback described above and to bring about a protection method allowing an optimum performance of the connection even when the power levels of signals received through the working and redundancy paths are essentially different.
This objective is attained through the solution defined in the independent claims.
The idea of the invention is at each time to select as the working path the path where the arriving signal has the higher power level. The power level difference, which the signals must have, to be considered to be of different magnitude, may vary. What is essential, however, is that the current power level of the signal received from the working path will not affect the selection (otherwise than through the said power level). In other words, the power level of the said signal may be within the permissible range, but the exchange is made nevertheless.
Owing to the invention, the performance of a protected optical link is made to remain optimum with all the mutual power level combinations that signals arriving from different paths may have.